1. The Field of the Invention
The present invention relates to x-ray generating devices. More particularly, the present invention relates to an x-ray tube having an integral housing assembly that allows for improved performance, especially in x-ray mammography applications.
2. The Relevant Technology
X-ray devices are extremely valuable tools for use in a variety of medical applications. For example, such equipment is commonly used in areas such as diagnostic and therapeutic radiology; radiography is of particular use to diagnose breast cancers.
Regardless of the particular application involved, the basic operation of medical x-ray devices is similar. In general, x-rays, or x-ray radiation, are produced when electrons are produced and released, accelerated, and then stopped abruptly. Typically, this entire process takes place within a housing that defines an evacuated envelope; the housing is typically constructed of glass, metal, or a combination thereof. Three primary components are typically disposed within the evacuated envelope: a cathode, which produces the electrons; an anode, which is axially spaced apart from the cathode and oriented so as to receive electrons emitted by the cathode; and an electrical connection for allowing a voltage generation element to apply a voltage between the cathode and the anode to accelerate the emitted electrons.
In operation, a voltage potential is applied between the cathode and the anode. This causes the electrons that are emitted from the cathode filament to form a thin stream or beam, and accelerate to a very high velocity towards target surface positioned on the anode. This target surface (sometimes referred to as the focal track) is comprised of a refractory metal, so that when the electrons strike the target surface, at least a portion of the resulting kinetic energy is converted to electromagnetic waves of very high frequency, i.e., x-rays. The resulting x-rays emanate from the anode target surface, and are then collimated for penetration into an object, such as an area of a patient""s body. As is well known, the x-rays that pass through the object can be detected and analyzed so as to be used in any one of a number of applications, such as a medical diagnostic examination.
In general, a very small part of the input energy results in the production of x-rays. A majority of the kinetic energy resulting from the electron collisions at the target surface is converted into heat of extremely high temperatures. The heat is absorbed by the anode and is conducted to other portions of the anode assembly, and to the other x-ray tube components that disposed within the evacuated envelope housing. Over time, this heat can damage the anode, the anode assembly, and/or other tube components, and can reduce the operating life of the x-ray tube and/or the performance and operating efficiency of the tube.
Several approaches have been used to help alleviate problems arising from the presence of these high operating temperatures. For example, in some x-ray devices the x-ray target, or focal track, is positioned on an annular portion of a rotatable anode disk. The anode disk (also referred to as the rotary target or the rotary anode) is then mounted on a supporting shaft and rotor assembly, that can then be rotated by some type of motor. During operation of the x-ray tube, the anode disk is rotated at high speeds, which causes the focal track to continuously rotate into and out of the path of the electron beam. In this way, the electron beam is in contact with any given point along the focal track for only short periods of time. This allows the remaining portion of the track to cool during the time that it takes to rotate back into the path of the electron beam, thereby reducing the amount of heat absorbed by the anode.
While the rotating nature of the anode reduces the amount of heat present at the focal spot on the focal track, a large amount of heat is still present within the anode, the anode drive assembly, and other components within the evacuated housing. This heat must be continuously removed to prevent damage to the tube (and any other adjacent electrical components) and to increase the x-ray tube""s efficiency and overall service life.
One approach has been to place the housing that forms the evacuated envelope within a second outer metal housing, which is sometimes referred to as a xe2x80x9ccan.xe2x80x9d This outer housing or can serves several functions. First, it acts as a radiation shield to prevent radiation leakage. As such, it must be at least partially constructed from some type of dense, x-ray absorbing metal, such as lead. Second, the outer housing serves as a container for a cooling medium, such as a dielectric oil, which is can be continuously circulated by a pump over the outer surface of the inner evacuated housing. As heat is emitted from the x-ray tube components (anode, anode drive assembly, etc.), it is radiated to the outer surface of the evacuated housing, and then at least partially absorbed by the coolant fluid. The heated coolant fluid is then passed to some form of heat exchange device, such as a radiative surface, and the heat is removed. The fluid is then re-circulated by the pump back through the outer housing and the process repeated.
The dielectric oil (or similar fluid) can be used to serve functions other than cooling. For example, the oil serves as an electrical insulator between the inner evacuated housing, which contains the cathode and anode assembly, and the outer housing, which is typically comprised of a conductive metal material.
While useful as a heat removal medium and/or as an electrical insulator, the use of oil and similar liquids can be problematic in several respects. For example, use of a fluid adds complexity to the construction and operation of the x-ray generating device in several areas. First, use of fluid requires that there be a second outer housing or can structure to retain the fluid. This outer housing is constructed of a material that is capable of blocking x-rays, and it must be large enough to be completely disposed about the inner evacuated housing and allow fluid to be disposed therein. This increases the cost and manufacturing complexity of the overall device. Also, the outer housing requires a large amount of physical space, resulting in the need for an overall larger x-ray generating device. Similarly, the space required for the outer housing reduces the amount of space that can be utilized by the inner evacuated housing, which in turn limits the amount of space that can be used by other components within the x-ray tube. For example, the size of the rotating anode is limited; a larger diameter anode is desirable because it is better able to dissipate heat as it rotates.
Moreover, construction of the outer housing adds expense and manufacturing complexity to the overall device in other respects. If the liquid is used as a coolant, the device may also be equipped with a pump and a radiator and the like, that in turn must be interconnected within a closed circulation system via a system of tubes and fluid conduits. Also, since the oil expands when it is heated, the closed system must provide a facility to expand, such as a diaphragm or similar structure. Again, these additional components add complexity and expense to the x-ray device""s construction. Moreover, the tube is more subject to fluid leakage and related catastrophic failures attributable to the fluid system.
The presence of a liquid coolant/dielectric is also detrimental because it does not function as an efficient noise insulator. In fact, the presence of a liquid may tend to increase the mechanical vibration and resultant noise that is emitted by the operating x-ray tube. This noise can be distressing to the patient and/or the operator. The presence of liquid also limits the ability to utilize other, more efficient materials for dampening the noises emitted by the x-ray tube due to space restrictions and the need for effective electrical insulation.
Some prior art x-ray tubes have eliminated the use of an outer housing and fluid as a coolant/dielectric medium. For example, some solutions utilize forced air to remove heat from the evacuated housing and its components. However, these approaches have not been entirely satisfactory for a variety of reasons. Also, proposed solutions are not well suited for certain types of x-ray applications, such as x-ray mammography.
For example, known x-ray generating devices that utilize forced air as a cooling medium are adapted for high voltage x-ray applications; such applications typically utilize a 150 kV operating potential, or higher, between the anode and cathode. High operating voltages result in higher operating temperatures, and to ensure sufficient heat removal with air convection, these x-ray tubes typically are equipped with fins, or channels formed on the outer surface of the evacuated envelope so as to enhance heat removal. As with previous solutions, this need for additional structure increases manufacturing complexity, and requires additional physical space requirements for the assembly. Moreover, in these types of devices, since the outer housing is eliminated, the housing forming the evacuated enclosure must provide a sufficient level of radiation shielding. To do so at such higher operating voltage levels, the walls that form the enclosure must either be very thick, or must be constructed of more expensive materials. Again, this requires increased physical space and/or results in higher manufacturing costs.
In addition to the increased shielding capacity that must be provided by the walls of the single housing forming the evacuated enclosure, prior art devices must also provide additional shielding within the enclosure itself. For instance, openings are typically provided through the top and bottom portions of the evacuated housing, for example, to allow for the passage of electronic wires to the cathode assembly. Additional shielding structure must be provided so as to block any x-rays from excaping through these openings. Again, this adds to the amount of physical space that is available to other components, and increases manufacturing complexity of the x-ray tube.
Radiographic devices utilizing air cooling must also replace the dielectric oil as the means for electrically insulating the evacuated envelope (the cathode and the anode) from the rest of the assembly. Also, the device must provide for some facility for reducing the amount of noise emitted by the x-ray tube during operation. As previously noted, the occurrence of noise resulting from a rotating anode can be especially troublesome to patients during some applications, such as mammography applications. However, the use of ceramic insulators and the like that are used in known devices do little to reduce operating noise, and thus have not been entirely satisfactory.
Thus, what is needed in the art, is a radiographic device that does not require the use of an outer housing for containing oils or similar fluids for the removal of heat and/or for providing a electrical insulator. Such a device would thereby eliminate the liabilities associated with the use of such fluids, such as increased manufacturing complexity, potential for failure and need for increased physical space. Moreover, the device should be especially suited for lower energy applications, such as mammography. The device should also preferably maintain safe levels of radiation containment, and should also emit low amounts of audible noise during operation.
It is therefore a primary object of the present invention to provide an x-ray generating device that eliminates the need for multiple housings, and instead utilizes a single integral evacuated housing for containing the components of the x-ray tube.
Another objective of the present invention is to provide an integral evacuated housing that is especially suitable for use in connection with low power radiation applications such as mammography.
A related objective of the present invention is to provide an x-ray generating device that is reduced in size, and that utilizes a fewer number of components so as to have reduced manufacturing costs and increased reliability.
Another objective of the present invention is to provide an x-ray generating device that utilizes an integral evacuated housing assembly for enclosing the anode and cathode assembly that provides sufficient levels of radiation shielding and limits the amount of radiation leakage to acceptable levels.
A related objective is to provide an x-ray generating device that utilizes an integral. evacuated housing assembly for enclosing the anode and cathode assembly that acts as a heat transfer element for transferring heat away from the anode and anode assembly.
Yet another objective of the present invention is to provide an x-ray generating device that utilizes an integral evacuated housing assembly that can be air cooled without the need for fins or similar structure to transfer heat from the anode assembly to the air coolant.
Still another objective of the present invention is to provide an x-ray generating device that emits a low operating noise.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. Briefly summarized, the present invention is directed to an x-ray generating apparatus that is particularly useful for use in low power x-ray applications, such as mammography. The apparatus eliminates the need for an outer housing, and instead utilizes a single integral housing assembly for providing the vacuum enclosure that contains the cathode and anode assemblies. In preferred embodiments, the x-ray generating apparatus of the present invention is particularly adapted for use in low power applications, where the energy potential between the anode and the cathode is approximately 25-30 kV, with an operating current at approximately 80-100 mA. These lower kV levels produce x-rays that have a lower energy spectrum, and the lower energy x-rays are better absorbed by softer breast tissue, resulting in an overall better contrast in the resulting x-ray image.
In preferred embodiments, the single integral housing is formed as a generally cylindrically shaped body. Supported on a cathode mounting structure within the interior of the housing is a cathode having an emission source for emitting electrons. The cathode is supported so as to be positioned opposite from a focal track formed on a rotating anode. The focal track is positioned on the anode so that x-rays are emitted through a window formed through the side of the housing. In addition, the cathode is freely supported on the cathode mounting structure, insofar as it is supported without the use of an oversized radiation shield or disk for blocking x-rays from exiting an opening formed through the housing. The elimination of a need for a cathode blocking disk frees up space within the interior of the housing, and reduces manufacturing complexity.
In a preferred embodiment, at least a portion of the integral housing is formed of low cost material such as copper, or a copper-like material, that possesses thermal conduction characteristics that allow heat to be absorbed from the anode assembly during operation, and then conducted to the outer surface of the integral housing. Also, that portion of the housing that is adjacent to the rotating anode includes walls that are of sufficient thickness so as to block x-rays, so as to comply with applicable FDA requirements. When used in a lower power mammography application, the x-rays are of relatively lower intensity, and thus the wall thickness needed to shield x-rays is relatively lowxe2x80x94even with copper. Again, this reduces the overall size of the integral housing, as well as its cost.
Preferred embodiments of the present invention utilize a forced air convection system to remove heat that is transferred to the outer surface of the integral housing, and to remove heat emitted from the stator, or motor assembly that is used to rotate the anode. This eliminates the need for coolant fluids, such as dielectric oil and the like, and therefore eliminates the problems inherent with the use of such fluids. In one embodiment, a fan is used to direct air over the outer surfaces of the integral housing; preferably the air flow is directed with an air flow shell that is disposed about at least a portion of the integral housing. Moreover, the heat transfer characteristics of the integral housing, together with the airflow, provides sufficient heat transfer such that integral housing does not require fins, channels, or other similar means for conducting heat away from the surface. This too reduces manufacturing complexity, and reduces the overall physical size of the evacuated housing.
Presently preferred embodiments of the present invention also include means for insulating the evacuated housingxe2x80x94both in an electrical sense and in an audible noise sense. In one embodiment, a dielectric gel is disposed between the integral housing and points external to the housing. The gel provides two functions: it electrically insulates the high voltage connection to the anode assembly, thereby preventing arching and charge up of the evacuated integral housing (especially the glass portion). Moreover, the gel acts as a damping material and absorbs vibration and noise that originates from the anode rotor assembly. Reduced noise emissions are especially important to maintain the comfort of the patient and to help reduce any anxiety that would otherwise result from high noise emissions.